1. Field of the Invention
The present invention generally relates to a wireless communication system supporting relaying. More particularly, the present invention relates to an apparatus and method for increasing a capacity gain without adding time resources in a wireless communication system.
2. Description of the Related Art
In a wireless communication system, Mobile Stations (MSs) located at a service area boundary or in a shadowing area may suffer from a poor channel condition. Accordingly, the wireless communication system may adopt a relay scheme using Relay Stations (RSs) to improve the link performances of such MSs. The relay scheme provides high-rate data channels to the MSs, thereby increasing overall system performance and expanding service coverage.
FIG. 1 illustrates the configuration of a conventional wireless communication system supporting relaying.
Referring to FIG. 1, an MS 110 within the service area 101 of a BS 100 is connected to the BS 100 by a direct link. An MS 120, which is outside the service area 101 of the BS 100 and thus has a poor channel condition, is connected to the BS 100 by a relay link established with an RS 130.
Thus, via the RS 130, the BS 100 can communicate with MSs that otherwise have a poor channel condition because they are outside the service area 101 of the BS 100 or because they are in a shadowing area and suffering from a shielding effect due to buildings.
The wireless communication system supports relaying in three methods.
One of them is Time Division Duplexing (TDD) relaying as illustrated in FIG. 2.
FIG. 2 illustrates the configuration of a conventional broadband wireless communication system supporting TDD relaying.
Referring to FIG. 2, the wireless communication system supports relaying using four independent time resources. The time resources include two uplink time resources and two downlink time resources. For example, a BS 201 transmits a signal {tilde over (S)}1 to an RS 203 in first time resources in step 211 and the RS 203 relays the received signal {tilde over (S)}1 to an MS 205 in second time resources in step 213.
After receiving the signal {tilde over (S)}1 from the RS 203, the MS 205 transmits a signal {tilde over (S)}2 to the RS 203 in third time resources in step 215 and the RS 203 relays the received signal {tilde over (S)}2 to the BS 201 in fourth time resources in step 217.
Another relaying method is coded bi-directional relaying, as illustrated in FIG. 3.
FIG. 3 illustrates the configuration of a conventional wireless communication system supporting coded bi-directional relaying.
Referring to FIG. 3, the wireless communication system supports relaying using three independent time resources. For example, a BS 301 transmits a downlink signal {tilde over (S)}1 to an RS 303 in first time resources in step 311 and an MS 305 transmits an uplink signal {tilde over (S)}2 to the RS 303 in second time resources in step 313. The downlink and uplink signals {tilde over (S)}1 and {tilde over (S)}2 from the BS 301 and the MS 305 are code symbols.
The RS 303 detects and decodes the code symbols {tilde over (S)}1 and {tilde over (S)}2. In step 315, the RS 303 performs an exclusive OR (XOR) operation of the symbols {tilde over (S)}1 and {tilde over (S)}2 and transmits the resulting symbol {tilde over (S)}3={tilde over (S)}1⊕{tilde over (S)}2 to the BS 301 and the MS 305 in third time resources.
The BS 301 can detect the uplink signal {tilde over (S)}1 from the signal {tilde over (S)}3 received from the RS 303 by performing an XOR operation of {tilde over (S)}3 with {tilde over (S)}1. The MS 305 can also detect the downlink signal {tilde over (S)}1 from the signal {tilde over (S)}3 received from the RS 303 by performing an XOR operation of {tilde over (S)}3 with {tilde over (S)}2.
The other relaying method is multi-user and Spatial Division Multiple Access (SDMA) relaying, as illustrated in FIG. 4.
FIG. 4 illustrates the configuration of a conventional wireless communication system supporting multi-user SDMA relaying.
Referring to FIG. 4, the wireless communication system supports relaying using two independent time resources. For example, a BS 401 and an MS 405 transmit signals {tilde over (S)}1 and {tilde over (S)}2 to an RS 403 in first time resources in 411 and the RS 403 relays the signals {tilde over (S)}1 and {tilde over (S)}2 to the MS 405 and the BS 401, respectively in second time resources in step 413. That is, the RS 403 can detect the signals {tilde over (S)}1 and {tilde over (S)}2 by a multi-user detection scheme used for a multi-antenna system, considering that the BS 401 and the MS 405 are multi-users. The RS 403 can then transmit a signal {tilde over (S)}3 which includes the signals {tilde over (S)}1 and {tilde over (S)}2 as given by
            S      ~        3    =                    [                                                            H                T                                                                        G                                      ]                    -        1              ⁡          [                                                                  S                ~                            2                                                                                          S                ~                            1                                          ]      to the MS 405 and the BS 401 in the same time resources.
As described above, relaying in the wireless communication systems improves the link performances of MSs at a cell boundary and expands coverage.
However, the TDD relaying consumes twice as many resources as the multi-user SDMA relaying. The coded bi-directional relaying does not maximize an overall system capacity gain due to the use of three independent time resources. The multi-user SDMA relaying also has a shortcoming in that, since an RS treats a BS and an MS as multi-users and thus the BS and the MS share spatial resources of the RS, the number of streams exchanged between the BS and the MS is limited by the number of RS antennas. The multi-user SDMA relaying also increases the complexity of the RS because the RS uses the multi-user detection scheme of the multi-antenna system. Moreover, if the BS, the MS and the RS each have a single antenna, the multi-user SDMA relaying is not viable.